Colour and Chemical Constitution. 
127 
C6H5O 
This on hydrolysis gives _^-aminophenol and HO • the tauto- 
O^CO 
meric phthalein-forni of ^-oxybenzoylbenzoic acid (see beginning). 
The new phthalein obtained from this acid alone is pinkish- violet in 
alkali and has a rather broad absorption-band varying between A 559 and 
\ 561 as the alkali strength is increased ; excess eventually bleaches it. The 
H0SO4 band is at X 501. It is thus scarcely distinguishable from the 
supposed o-^-plienolphthalein just described, and only differs from ordinary 
phenolphthalein in a slight degree, having a spectrum similar to that of 
phenol-o-cresolphthalein. The further investigation of this interesting 
substance must be postponed to another opportunity. 
A new property of the phthaleins has been discovered. 
It is possible to find a particular considerable strength of alkali in which 
any phthalein gives a colourless solution in the cold, but which becomes 
coloured on heating to near the boiling-point and again fades on cooling and 
allowing to stand For ordinary phenolphthalein the alkali-strength 
required is slightly over N. strength. Tetriodophenolphthalein requires 
much weaker alkali to exhibit the phenomenon, and a-uaphtholphthalein 
requires about 2N alkali. I cannot suggest any explanation of this, as it is 
not likely that the hydroxide-ion concentration can be much altered by heat 
when it is already so high to begin with. 
The following is an addition to Part II of this series : 
(1) Phenoljjhthalein mono-o-carhoxylic acid made by condensation of 
oxybenzoylbenzoic acid with salicylic acid is faintly coloured pink by 
ammonia (absorption-band \ 561). It is coloured deep violet-pink by 
caustic alkali (absorption-band X 567). This substance promises to be a 
valuable indicator, being like thymolphthalein in properties (i. e. suitable 
for determination of magnesia, etc.), but of a more favourable colour. 
(2) The corresponding dicarhoxylic acid (from phthalic and salicylic 
acids) described in Part I, has even more marked insensitiveness to alkali, 
having less affinity than Poirrier-blue. It will be very useful for proving 
the presence of definite caustic alkalinity at about N/100. 
(3) Phenoldi]jhenylami7iephthalein is also not affected by ammonia ; it is 
violet in soda (absorption-band X 565 and dulling of centre). It exhibits 
the same colour in 50 per cent. HgSO^, but is green in cone. 113804. 
Phenoldimethylanilinephthalein does not show the same colour in 50 
per cent. H2SO4 as in alkali. 
Attempts to make mono-ortho-oxy-derivatives of diphenylphthalide by 
condensing benzoylbenzoic acid with para-substituted phenols gave products 
which were yellow in alkali and therefore probably not of the expected 
nature. 
10 
